Swaging, mechanically squeezing a fitting onto a tube, is a known technique for joining mechanical fittings onto tubes found in high pressure fluid systems. The construction of high pressure fluid systems, those operating above one hundred psig, often requires use of such mechanical fittings to join tube sections together in a mechanical swaging process in circumstances in which other techniques, such as welding, brazing, or soldering, cannot be used. In the aerospace field a variety of different mechanical fittings are available for that application and, while relatively impervious, all leak to some slight degree.
Typically the smallest leak rate achieved with such mechanical fittings, as swaged to the fluid line and using Helium as the pressurizing medium, is 10.sup.-6 cubic centimeters per second. It is found that the leakage is even greater when the outside of the tube lines are covered with a hard metal plating material, such as nickel or chrome. In the latter case good seals with leak rates of 10.sup.-6 cubic centimeters per second, or less, cannot be achieved, which is believed to be principally due to the inability of the hard metal plating to deform during the fitting swaging process. Although a leak rate of 10.sup.-6 cc per second is acceptable in practice, it is found in aircraft, as example, after some period in service, the leakage rate of some fittings increases, often rising to unacceptable levels. That increase may be attributed to the harsh aircraft environment, particularly, the adverse effect of temperature cycling, sustained or cyclic pressure, and vibration acting on the fittings.
Fluid leaks in those aircraft fluid systems are repaired when the aircraft returns to base. Such leaks often prove difficult and costly to repair, due to having poor access to the location of the leak in the aircraft. For permanent space station application locating and repairing a major leak in a fluid system requires major effort; a situation that should be avoided if at all possible. The obvious manner of avoiding that difficult situation is to eliminate the use of mechanical fittings in fluid systems for space application, a prohibitively expensive alternative and, hence, impractical. Welding is not as yet an acceptable practice for making repairs in the outer space environment. As a consequence, mechanical fittings remain a practical necessity both for initial fabrication and repair.
The present invention solves the problem of achieving the required low leak rate standard for fluid lines, by providing significantly lower initial leakage rates than is available with prior constructions of mechanical fittings. The improvement is achieved with a slight inexpensive modification to the structure of the standard mechanical fittings and/or tubing. The improved sealing is attained on nickel, chrome or any other hard plated metal surface as well as on the conventional titanium, aluminum, stainless steel, inconel and monel tubing.
As further background it is noted that the use of welding as a means of providing an impervious connection or joint between tubes in high pressure systems is a known alternative coupling or joining technique. However, the present invention is premised on the avoidance of any welds or welding process. Welding is avoided as a repair technique in difficult to work environments. The use of wet solder is also known as a means to join tubing together. However, although soldered joints are useful in low pressure water and gas plumbing systems, they fail in high pressure fluid systems. Even though the principal advantage of the invention and motivation for the invention is in high pressure hydraulic and fluid systems, it is recognized that the invention is not so limited in application. It may also be employed in low pressure systems as an alternative to existing joining techniques.
An object of the invention, therefore, is to provide a mechanical seal of high integrity for high pressure fluid systems;
Another object is to provide a high pressure mechanical seal that is applied by a swaging process;
A further object of the invention is to convert existing mechanical fittings, found to have leak rates of 10.sup.-6 cubic centimeters per second or less, as measured using Helium, into higher quality fittings that have leak rates of only 10.sup.-8 cubic centimeters per second or less; and
An additional object of the invention is to provide a mechanical seal structure which exhibits leakage that does not exceed a leakage rate on the order of 10.sup.-8 cubic centimeters per second or better without the necessity and expense of materially changing the mechanical design and size of existing fittings and tubes.